1. Field of the Invention
The present invention relates to an optical pickup apparatus that can record information on and reproduce information from an optical disc including a plurality of layers of information recording surfaces.
2. Description of the Related Art
Optical pickup apparatuses are known, which can record information on and reproduce information from an optical disc including a plurality of layers of information recording surfaces. Specifically, this pickup apparatus includes two lenses which are an objective lens and a spherical aberration correction lens. The objective lens is a lens for applying the laser beam for recording or reproducing information to a plurality of layers of information recording surfaces, selectively, where the surfaces are included in an optical disc mounted on a turn table. The spherical aberration correction lens is a lens capable of moving on a light path between the objective lens and a light source of the laser beam to correct spherical aberration that may occur on application of the laser beam to any one of information recording surfaces from the objective lens.
For example, a description will be given with respect to an example of a spherical aberration correction in the case of focusing the laser beam on each of two layers (L0 and L1 layers) of the information recording surfaces in an optical disc such as Blu-ray Disc (registered trademark), etc., where it is assumed that a collimator lens, e.g., is used for the spherical aberration correction lens.
First, an arrangement for an optical system configured with the objective lens, the collimator lens, and a blue-violet semiconductor laser is set such that the spherical aberration becomes zero when a parallel laser beam (parallel light) generated by the blue-violet semiconductor laser and the collimator lens is focused, through the objective lens, on a virtual layer (Lm layer) located midway between the two layers of the optical disc.
In order to focus the laser beam on an L0 layer that is a layer with a thicker protective layer between the two layers, a diverging laser beam (divergent light) is generated by the blue-violet semiconductor laser and the collimator lens to enter the objective lens. Specifically, in the optical system for achieving focus on the above Lm layer, the spherical aberration can be made zero by moving the collimator lens away from the objective lens toward the blue-violet semiconductor laser along the light axis direction for a predetermined distance.
In order to focus the laser beam on an L1 layer that is a layer with a thinner protective layer between the two layers, a converging laser beam (convergent light) is generated by the blue-violet semiconductor laser and the collimator lens to enter the objective lens. Specifically, in the optical system for achieving focus on the above Lm layer, the spherical aberration can be made zero by moving the collimator lens toward the objective lens away from the blue-violet semiconductor laser along the light axis direction for a predetermined distance.
In general, in the case of an optical disc having a plurality of layers of information recording surfaces, again, the spherical aberration when the laser beam is focused on the information recording surface of each layer, can be made substantially zero by moving the collimator lens to change the degree of divergence of the laser beam incident on the objective lens. That is, to so-called multilayer optical discs, a well-known method is applicable, which is used for correcting spherical aberration depending on a difference in thickness of the protective layer between CD (Compact Disc) and DVD (Digital Versatile Disc), for example (see, e.g., Japanese Patent Application Laid-Open Publication No. 10-134400).
In general, when an arrangement of any lens is changed in an optical system configured with a plurality of lenses, the optical magnification of the optical system may change.
On the other hand, the optical magnification is considered to have a quantitative correlation with a rim intensity, where the rim intensity determines: the spot shape of the laser beam at the focal point; the optical coupling efficiency (e.g., a ratio of an amount of light incident on the objective lens out of an amount of light emitted from the blue-violet semiconductor laser) of the laser beam; and the like. The optical magnification represents, for example, a proportion of a size of an “image” at the focal point of the laser beam from the objective lens relative to a size of an “object” at the emitting point of the blue-violet semiconductor laser in the optical system. The rim intensity represents sharpness of intensity distribution on the cross section orthogonal to the light axis direction of the laser beam (e.g., sharpness increases as the rim intensity decreases).
For example, if the above spherical aberration correction is performed for each of the two layers (L0 and L1 layers) of the information recording surfaces in the optical disc, the optical magnification in the case of focusing the laser beam on the L0 layer generally becomes different from the optical magnification in the case of focusing the laser beam on the L1 layer, due to the displacement of the collimator lens. Characteristics of recording or reproducing information therefore become different between in the L0 layer and in L1 layer, which necessitates the inclusion of a configuration capable of accommodating the characteristic differences in a processing circuit, firmware, etc., for recording or reproducing information, of the optical pickup apparatus. Specifically, for example, it is necessary that the emitting power of the blue-violet semiconductor laser, the recording pulse waveform for recording information, and the like are adapted to be switchable between the L0 and L1 layers. This may cause an increase in complexity and costs in the optical pickup apparatus.
The optical magnification is also considered to have a correlation with the stability of the tracking control exercised by a differential push-pull method. The differential push-pull method is a method by which the tracking control is exercised: through the application of 0th order light to a track that is a target of recording or reproducing information in the optical disc; and at the same time, through the point-symmetrical application of +1st order and −1st order diffracted lights with respect to the 0th order light, wherein the 0th order light and the ±1st order diffracted lights are obtained by diffracting the laser beam with a diffraction grating, etc. The intervals among three spots composed of once spot formed by the 0th order light and of two spots formed around the track by the ±1st order diffracted lights, are varied depending on the optical magnification.
For example, if the above spherical aberration correction is performed for each of the two layers (L0 and L1 layers) of the information recording surfaces in the optical disc, the optical magnification in the case of focusing the laser beam on the L0 layer generally becomes different from the optical magnification in the case of focusing the laser beam on the L1 layer, due to the displacement of the collimator lens. The intervals among the three spots therefore become different between in the L0 layer and in the L1 layer, which may cause the variations in the amplitude of the tracking error signal between the two layers, resulting in the instability of the tracking control.
Therefore an object of the present invention is to provide a low-cost optical pickup apparatus capable of performing easily spherical aberration correction while maintaining characteristics of information recording or reproducing and stability of the tracking control substantially constant, for each of a plurality of layers of information recording surfaces in an optical disc.